Direct and indirect effects of terrain, snow, and shrubs on the structure of an alpine herbaceous community in the Canadian Rocky Mountains

Alpine ecosystems have extreme environments that limit plant growth, favoring stress-tolerant species. Climate change has resulted in the expansion of shrubs at the expense of poor competitors, including herbaceous species. Here, we explore the pathways linking local terrain features, snowmelt dynamics, and shrub cover on herbaceous plant cover and species richness at Cardinal Divide in the Canadian Rocky Mountain Front Ranges using a series of structural equation models (SEMs) that isolate direct from indirect effects among factors. We measured the biotic and abiotic conditions within 338 quadrats (0.25-m ² ) across a gradient of environments typical of the open and expanding krummholz alpine sites on the divide. Herbaceous cover and richness were negatively related to shrub cover but positively associated with late-snow persistence and terrain features that supported higher winter snow accumulation and greater soil depth. Dwarf deciduous shrubs were minimally affected by differences in the timing of snowmelt. Tall evergreen shrubs were uncommon but abundant where present, substantially reducing herbaceous cover and richness. The indirect effects of shrub cover tempered the positive association of snow on herbaceous cover and richness. Concave terrain surfaces and cooler-facing slopes increased herbaceous composition, while soil depth supported taller shrubs and herbaceous communities. This study underscores the complex relationships, both direct and indirect, between the abiotic and biotic environments of the alpine plant community.